At the high gas temperatures at which gas turbines of the modern generation are operated, it is essential for the guide and moving blades of the turbine that are used there to be cooled. To this end, a gaseous cooling medium is used, for example a compressed air quantity branched off from the compressor of the gas turbine at a suitable point, and/or sometimes steam, in particular if the gas turbine is part of a combined-cycle power plant, or otherwise when steam is available in suitable quality and quantity. This cooling medium is directed through cooling channels arranged in the blade and frequently running in serpentine shapes, this being effected via an open and/or closed cooling path. The cooling medium is also often discharged outward through corresponding openings (holes, slots) at various points of the blade in order to achieve a cooling effect, in particular on the outer side of the blade, by the film cooling forming there. An example of such a cooled blade is described and shown in the publication U.S. Pat. No. 5,813,835.
Within the scope of blade cooling, the trailing edge of the blade must also often be subjected to cooling by cooling medium being discharged through a slot-shaped opening arranged in front of the trailing edge and running substantially parallel to the trailing edge, the cooling medium then sweeping over the trailing edge and that region of the blade surface which lies between the opening and the trailing edge. Such cooling of the trailing edge is shown in FIG. 3 of U.S. Pat. No. 5,813,835 with the reference numerals 208 and 210.
The basic geometry of the trailing edge cooling is reproduced in a highly simplified form in FIG. 1. The blade 10, which extends in a longitudinal direction, that is to say in a radial direction with respect to the turbine axis, and which ends in a blade tip 12, has a leading edge 11 upstream and a trailing edge 13 downstream. Between leading edge 11 and trailing edge 13, the blade, with a wing profile, forms a pressure side 23 and a suction side 24.
An exit slot 14 for a cooling medium (in particular cooling air) is provided on the pressure side 23 in front of the trailing edge 13 and runs parallel thereto, through which exit slot 14 the cooling medium discharges outward and sweeps as cooling flow 16 over the trailing edge 13. The cooling medium is fed to the exit slot 14 through a cooling channel 15 in the interior of the blade 10. Arranged in a distributed manner in the longitudinal direction in the exit slot 14 are control elements 17, through which firstly the cross-sectional area of the exit slot 14 is reduced (that is to say controlled) and secondly the cooling medium is distributed over the entire length of the exit slot 14.
The significance of the control elements 17 for the control of the cooling flow 16 and thus also for the efficiency of the gas turbine overall is the subject matter of another publication, namely US-A1-2005/0232770. Control elements (“control features”) of varying configuration in the exit slot are proposed in this publication, which control elements are intended to further reduce the cross-sectional area of the exit slot achievable by a casting process but at the same time also help to increase the mechanical stability of the slot region. Depending on the edge contour of the control elements 17, the cooling flow 16 in this case can be laminar or turbulent.
This reduction in the trailing edge cooling flow by obstacles or control elements arranged in the slot is always disadvantageous when the cooling flow 16 flows out with a moderate or low mass flow. In such a case, converging jet configurations 18 form in the cooling flow at the outlet of the exit slot according to FIG. 2, and generated in said jet configurations 18 are intermediate areas 19 which are cooled to a lesser extent and are thus subjected to higher temperatures. However, the reason for the low mass flow can be seen in the fact that recycled air is used.